The present invention relates to an elastic fender adapted to be mounted on the hull of a ship and a shore installation such as pier, wharf, quay, dock, float or like structure for cushioning the impact of a ship against a shore installation.
Various types of elastic fenders have long been used to cushion the impact of a ship against a shore installation for protecting both the side of the hull of the ship and the shore installation. Elastic fenders generally considered feasible for this purpose are of a type capable of exhibiting such a performance curve as to follow the following stages in sequence during elastic deformation of such fender.
(I) Initial Set-up Stage: During this stage, the rate of increase of the reaction force of the fender incident to compression of such fender which takes place as the hull of the ship contacts the fender is greater than the rate of increase of the amount of deformation of such a fender by the action of such compression.
(II) Set-back Stage: This set-back stage occurs subsequent to the initial set-up stage and, during this stage, the rate of increase of the reaction force of the fender being compressed is retarded to a value smaller than the rate of increase of the amount of deformation of such fender due to the fact that the fender is flexed or buckled.
(III) Final Set-up Stage: The rate of increase of the reaction force of the fender again becomes greater than the rate of increase of the amount of deformation of such fender due to the fact that the fender is further compressed in contact with the hull of the ship.
The performance of the elastic fender is evaluated in consideration of the amount of deformation of the fender, which occurs subsequent to the application of an impact from the ship to such fender, and the magnitude of energy absorbed by such fender, that is, the energy absorbability, which is, in a graph showing the above described performance curve, represented by the surface area of such a region as defined by the performance curve, one of the axes of coordinates representing the amount of deformation and a line drawn from the point of start of the final set-up stage on the performance curve and intersecting at right angles to such one of the axes of coordinates. Specifically, the elastic fender is considered excellent in performance if the deformability of the fender is high while exhibiting a large surface area of the above described region.
Some types of elastic fenders which exhibit the above described performance curve are disclosed in, for example, the British Patent Specification No. 945,456, published on Jan. 2, 1964; the U.S. Pat. Nos. 3,418,815 and 3,418,816, both patented on Dec. 31, 1968; and the U.S. Pat. No. 3,820,495, patented on June 28, 1974.
The above mentioned British Patent Specification discloses an elastic fender of a construction comprising a hollow rubber tube of trapezium-shaped cross section with a fitting surface, a buffer surface opposite the fitting surface and a pair of supporting walls between the fitting and buffer surfaces, the section of the tube in a plane at right angles to the longitudinal axis of the tube having the approximate form of an isosceles trapezium with its top forming said buffer surface, the bottom forming said fitting surface and the two lateral sides of equal length forming said supporting walls. This British Patent Specification further discloses that the fender of the above described construction can attain a maximum performance if the trapezium forming the cross sectional shape of the hollow rubber tube satisfies the following requirements: EQU A=H to 3H, B=0.2H to 0.8H, t=0.15H to 0.4H, and .theta.=45.degree. to 80.degree.
wherein H is the height as measured between the top and the bottom, A is the bottom width, B is the top width, t is the wall thickness of each of the supporting walls, and .theta. is the angle of each of the lateral sides relative to the bottom.
The U.S. Pat. No. 3,418,815 discloses a fender of a construction comprising a hollow elastic tube similar to that disclosed in the above mentioned British Patent Specification, but has a rigid plate embedded in the buffer surface portion for optimum load-distribution both on the side of the ship and throughout the elastic material to produce improved energy-absorbing characteristics. The use of an additional rigid plate embedded in the fitting surface portion of the hollow elastic tube is also disclosed therein.
The U.S. Pat. No. 3,418,816 discloses a fender assembly constituted by a plurality of fenders each being substantially identical with that disclosed in the above mentioned U.S. Pat. No. 3,418,815.
Furthermore, an elastic fender of a construction having a substantially V-shaped cross section is well known and is shown in FIG. 1 of the accompanying drawings in a cross sectional view. Referring now to FIG. 1, this known V-shaped fender comprises a pair of elongated supporting walls 1 and 2 of equal width connected integrally at one side edge with each other by a bridge wall 3, said bridge wall 3 having a flat buffer surface 4 facing in a direction remote from fitting flanges 5 and 6 which extend respectively from the other side edges of the associated supporting walls 1 and 2 in a direction away from each other and in parallel relation to the plane of the buffer surface 4. This fender is of one-piece construction made of an elastic material such as rubber.
In the construction described above, when a compressive force is applied from, for example, the hull of a ship to the flat buffer surface 4 in a direction perpendicular to the plane of the buffer surface 4 while the fender is mounted on the shore installation with the flanges 5 and 6 rigidly secured thereto by means of, for example, set bolts, the V-shaped fender undergoes elastic deformation in such a manner that the opposed portions of the fender where the supporting walls 1 and 2 are integrally connected to the bridge wall 4 expand outwardly with respect to each other with a substantially intermediate portion of the bridge wall 3 tending to outwardly protrude into a space between the supporting walls 1 and 2, substantially as shown by the single chain line X, and when that portions of the fender where the supporting walls 1 and 2 are intergrally connected to the bridge wall 3 are subsequently brought to respective positions outwardly of associated lines Z, each drawn at right angles to the plane of the fitting surface of the corresponding fitting flange 5 or 6 and passing through the fulcrum Q about which the corresponding supporting wall 1 or 2 can freely pivot during the deformation of the fender, substantially as shown by the double chain line Y, the supporting walls 1 and 2 are buckled outwardly with respect to each other with a relatively large amount of reaction forces consequently generated against the hull of the ship. The performance curve of this fender shown in FIG. 1 is shown by (A) in a graph of FIG. 2 of the accompanying drawings.
In the construction shown in FIG. 1, if the outward expansion of that portions of the fender where the supporting walls 1 and 2 are integrally connected to the bridge wall 3 and the protrusion of the substantially intermediate portion of the bridge wall 3 can be restrained for a substantial period of time as long as possible, the time at which the buckling occurs in the supporting walls 1 and 2 in the manner described above can be delayed for a corresponding period of time, which in turn results in improvement of the energy absorbability of the fender.
In addition, in the construction shown in FIG. 1, immediately after the supporting walls 1 and 2 have started their buckling, they continuously contact the hull of the ship with the area of contact of each supporting wall 1 or 2 to the hull of the ship increased and, therefore, the area of the fender on which a load is imposed by the ship gradually increases. When this load acting area of the fender increases to such an extent as to spread outwardly of the imaginary lines Z which extend at right angles to the fitting flanges 5 and 6 and pass through the associated fulcrums Q about which the respective supporting walls 1 and 2 can pivot during the deformation or elastic collapse of the fender occurring in the manner described above, the supporting walls 1 and 2 become completely buckled outwardly with respect to each other while the direction in which the load acts on the fender becomes parallel to any one of the imaginary lines Z passing through the respective fulcrums Q. This is the start of the final set-up stage (III) referred to hereinbefore and the reaction force generated by the fender so deformed or collapsed accordingly increases rapidly. Therefore, if the contact of the supporting walls 1 and 2 to the hull of the ship during the buckling of the supporting walls 1 and 2 is shirked while the load of the ship is made to act on the fender in a direction diagonally with respect to the imaginary lines Z, the possibility of the buckling of the supporting walls 1 and 2 can be retained for a substantially prolonged period of time, which in turn results in improvement of the energy absorbability of the fender.
The fender disclosed in the U.S. Pat. No. 3,820,495 is considered an improved version of the fender of the construction shown in FIG. 1 in terms of the energy absorbability and is similar in construction to that shown in FIG. 1 except for a fluted groove and a fluted rib which are provided in the bridge wall of the fender of the last mentioned U.S. Patent. In addition, the latter also discloses the use of rigid reinforcement plates each embedded in the corresponding fitting flange.
According to the last mentioned U.S. Patent, the fluted rib extending lengthwise of the fender and protruding into a space between the supporting walls from the inside surface of the bridge wall opposite to the buffer surface is so designed that, when the fender is extensively collapsed with the supporting walls outwardly buckled with respect to each other, said rib can abut on the shore installation to support the bridge wall together with the buckled supporting walls for absorbing further shock energy. On the other hand, the fluted groove extending lengthwise of the fender and defined in the buffer surface of the bridge wall opposite to the fluted rib, thereby dividing the buffer surface into buffer areas one on each side of the fluted groove, is so designed that, as the fender is elastically collapsed, the buffer areas on respective sides of the fluted groove move in a direction close towards each other, while slip takes place between the hull of the ship and each buffer area, thereby avoiding the possible outward expansion of that portions of the fender where the supporting walls are integrally connected to the bridge wall, such as occurring in the construction shown in FIG. 1, and hence delaying the time at which each of the supporting walls starts its buckling movement.
The fender of the construction disclosed in the last mentioned U.S. Patent can, in fact, exhibit an improved energy absorbability in view of the fact that the time at which each of the supporting walls starts its buckling movement can be delayed by the provision of the fluted groove. However, in practice, since the coefficient of friction between the buffer areas of the fender and the hull is not always fixed and varies depending on the type of ships and/or the surface condition of the buffer areas, the movement of the buffer area on respective sides of the fluted groove in a direction close towards each other which takes place as the fender is collapsed by the application of a load thereto from the ship is not always stable and is likely to be facilitated where such frictional coefficient is low while to be retarded where it is high. By way of example, though the fender disclosed in the last mentioned U.S. Patent generally have such a performance curve as shown by (B) in the graph of FIG. 2, this fender would give such a performance curve as shown by (C) in the graph of FIG. 2 where the friction drag is large and would give a performance curve approximating to the performance curve (A) where the friction drag is small. In other words, the fender of the construction disclosed in the last mentioned U.S. Patent involves such a disadvantage that not only does the performance tend to vary depending upon the friction drag developed between the buffer areas of the fender and the hull of the ship, but also the energy absorbability is still relatively low.